1. Field of the Invention
The present invention relates to a power amplifier having improved idle current controllability, reduced susceptibility to device variations and stable temperature characteristics.
2. Background Art
Power amplifiers using GaAs heterojunction bipolar transistors (GaAs-HBT) are presently used widely as power amplifiers for use in portable telephone in CDMA and other systems. FIG. 13 is a circuit diagram showing a conventional GaAs-HBT power amplifier. The area in the dotted frame in FIG. 13 corresponds to a GaAs chip. Circuit elements in the other area are formed of chip parts and conductor lines on a module substrate.
In FIG. 13, reference characters Tr1 and Tr2 denote a preceding-stage amplifying device and a following-stage amplifying device, respectively. Bias1 denotes a preceding-stage bias circuit for driving the preceding-stage amplifying device, and Bias2 denotes a following-stage bias circuit for driving the following-stage amplifying device.
Vc1 and Vc2 respectively denote collector power supply terminals of the preceding-stage and following-stage amplifying devices. Vcb denotes a power supply terminal for the bias circuits Bias1 and Bias2. Vref denotes a terminal through which a control voltage is applied to the bias circuits Bias1 and Bias2. IN denotes an RF signal input terminal. OUT denotes an RF signal output terminal. R1 to R4 denote resistors; C1 to C10, capacitors; L1 and L2, inductors; and L3 to L8, lines having particular electrical lengths and functioning as inductors. Recently, in ordinary cases, C1, C2, and L1 forming an input matching circuit and C3, C4 and L2 forming an interstage matching circuit are integrated on the GaAs chip for the purpose of reducing the module size.
FIG. 14 is a circuit diagram showing a conventional bias circuit. This bias circuit corresponds to the above-described preceding-stage bias circuit Bias1 or following-stage bias circuit Bias2. In FIG. 14, Vref denotes a terminal to which a control voltage is externally applied; Trb1 to Trb3, Trb7 and Trb8 denote GaAs-HBTs; Tr denotes an amplifying device; and Rb1 to Rb3 and Rb5 to Rb8 denote resistors.
An emitter follower circuit including Trb1 applies voltage to the base (input terminal) of the corresponding amplifying device Tr according to the control voltage. An RF signal input from a terminal RFin is input to the base of the amplifying device Tr via a capacitor C in an input matching circuit. The amplified RF signal is output from the collector of the amplifying device Tr to a terminal RFout.
This bias circuit operates so as to constantly maintain the idle current through the preceding-stage amplifying device and the following-stage amplifying device of the power amplifier with respect to changes in temperature (see, for example, Japanese Patent Laid-Open No. 2004-343244). The idle current is a bias current in the power amplifier when there is no RF input power.
FIG. 15 is a diagram showing input/output characteristics of a conventional CDMA HBT power amplifier. When the input power Pin is increased, the idle current Ictq is constant but the output power Pout increases and the total operating current Ict increases.
FIG. 16 is a diagram showing a distortion characteristic of the conventional CDMA HBT power amplifier. The distortion characteristic is expressed by the adjacent channel leakage ratio (ACLR). The ACLR increases with increase in the output power Pout. The output power Pout, power gain Gp and efficiency PAE when the output power increases determine whether the characteristic of the power amplifier is good or bad.
FIG. 17 is a diagram showing a probability distribution with respect to the output power of the power amplifier in a CDMA terminal in a suburb. The probability of low output about 0 dBm is the highest while the probability of the maximum output about 27 dBm is low (see, for example, B. Sahu and G. A. Rincon-Mora, “A high-efficiency linear RF power amplifier with a power-tracking dynamically adaptive buck-boost supply,” IEEE Trans. MTT vol. 52, No. 1, pp. 112-120, January 2004). It is, therefore, desirable to enable easily obtaining the desired distortion characteristic by limiting the idle current to a small value when the output power is low and increasing the idle current when the output power is high. For example, as shown in FIG. 18, the idle current Icq of the power amplifier can be limited to the least necessary value by changing the idle current Icq according to the desired output power Pout. However, this method requires a complicated control circuit.
For this reason, a method for making selection between a high idle current and a low idle current in an analog manner according to the output power has been adapted in many cases. This method can be easily implemented if a current mirror circuit or the like is used with enhancement-mode field-effect transistors. However, there are only a small number of documents disclosing a GaAs-HBT power amplifier in which the idle current is controlling in an analog manner.
FIG. 19 is a circuit diagram showing a conventional GaAs-HBT power amplifier in which the idle current is controlled in an analog manner (see, for example, Y.-W. Kim, K.-C. Han, S.-Y. Hong, J.-H. Shin, “A 45% PAE/18 mA quiescent current CDMA/PAM with a dynamic bias control circuit,” 2004 IEEE Radio Frequency Interated Circuit (RFIC) Symposium, 2004 Digest of Technical Papers, PP. 365-368). In FIG. 19, Tr denotes an amplifying device; Vc, a collector power supply terminal of the amplification stage constituted by the amplifying device Tr; Vcb, a collector power supply terminal of a bias circuit; Vref, a reference terminal to which a reference voltage is externally applied; and Vmod, a control terminal to which a control voltage is externally applied. Trb11 to Trb17 denote GaAs-HBTs. Rb11 to Rb17 denote resistors. L denotes a transmission line. C denotes a capacitor. An RF signal input from a terminal RFin is input to the base of the amplifying device Tr via the capacitor C in an input matching circuit. The amplifying device Tr amplifies the RF signal and outputs the amplified RF signal from the collector to a terminal RFout. A base circuit formed by Trb11 to Trb17 and Rb11 to Rb17 applies a voltage to the base of the amplifying device Tr according to the control voltage applied to the terminal Vmod.
The power amplifier shown in FIG. 19 has a problem that the control voltage and the idle current are in a nonlinear relationship with each other and, therefore, the controllability of the idle current is low. The amplifier also has a problem that since the idle current control function and the biasing function are incorporated in one circuit, its susceptibility to device variations is high and it is difficult to make constant the temperature characteristic of the idle current (variation in idle current with temperature with respect to the same control voltage).